The functional role of mast cells, which are diffusely dispersed in the brain and aggregate in larger numbers in paraventricular regions, is still disputed. These cells are known to contain several biochemically active substances enabling them to prevent coagulation, control blood circulation, and take an active part in transmission of nerve impulses and in neurovegetative functions. On the basis of morphologic characteristics, the mast cells are considered to be in a labile stage of activity which can progress to exhaustion and cell death. By a quantitative study of such cell forms, indirect evidence of interaction between these cells and nervous tissue elements may be established. To this end, electron microscopic and quantitative studies have been initiated.